1. Field of the Invention
This invention relates to a pneumatic radial tire for motorcycle, and more particularly to a pneumatic radial tire for use in a front wheel of a motorcycle simultaneously establishing excellent durability and steering stability by a proper combination of a tread pattern and a belt structure.
2. Description of Related Art
This type of the conventional tire comprises a belt containing cords extended substantially in a circumferential direction of a tread and has a directional tread pattern defined by main slant grooves each having an acute-side inclination angle of not less than 45.degree. with respect to the circumferential direction of the tread and extending from a tread center up to a tread shoulder region. The tread pattern has main slant grooves 1, 2 convexly curing toward a rotating direction of the tire on both sides of a tread center C which are arranged symmetrically with respect to the tread center C as shown in FIG. 1.
In such a tire and hence such a tread pattern, there is a problem that shimmy is liable to be particularly caused in a front wheel tire due to a fact that the bending rigidity of the tread is high in a meridional section of the tire. Also, circumferential components of the main slant grooves 1, 2 are less, so that there are problems that drainage performance on wet road, resistance to lateral slipping and the like are low and a high gripping force on wet road surface can not be developed.
On the contrary, if it is intended to adjust the bending rigidity of the tread by arranging circumferential grooves continuing straightforward in the circumferential direction of the tread on a ground contact region of the tread in addition to the main slant grooves 1, 2, the drainage performance on wet road can be enhanced while controlling the occurrence of shimmy. However, since the cords in the belt substantially extend in the circumferential direction of the tread, the bending resistance at the bottom of the circumferential groove becomes small and hence there is a problem that cracking at the groove bottom is created at a relatively premature stage.
As partly and collectively shown in FIG. 2, therefore, there are proposed patterns in one wherein circumferential zigzag grooves 3 continuously extending zigzag in the circumferential direction of the tread are arranged on the ground contact region of the tread in addition to the main slant grooves 1, 2. In another pattern slant sub-grooves 4 or 5 are arranged to extend straightforward and intersect with the main slant groove 1, 2 or open to the main slant grooves 1, 2. In another pattern slant sub-grooves 6 extend straightforward between the main slant grooves 1, 2 so as not to communicate therewith and have a large inclination angle .alpha. of more than 30.degree. with respect to the circumferential direction of the tread.
According to these patterns, the control of shimmy and the improvement of drainage performance on wet road are generally attained owing to the presence of the circumferential zigzag groove 3, or slant sub-groove 4, 5 or 6.
However, when the circumferential zigzag groove 3 is arranged to continuously extend in the circumferential direction of the tread, there is an improving effect as compared with the straight groove, but the rigidity is lowered too much to degrade the stability and the steering property.
When the slant sub-grooves 4, 5 are arranged to intersect with or open to the slant main grooves 1, 2, the rigidity is lower than that of the circumferential zigzag groove 3 and the stability and the steering property are further degraded.
Even in both cases, the occurrence of cracking at the bottom of the zigzag groove 3 or the sub-groove 4 or 5 can not sufficiently be prevented.
When the slant sub-grooves 6 having an inclination angle .alpha. of more than 30.degree. with respect to the circumferential direction of the tread are arranged between the slant main grooves 1, 2, the drainage performance on a wet road can not be improved to an expected extent. Also, the component of the sub-groove 6 in the circumferential direction of the tread is less and the length of groove edge in the circumferential direction of the tread is short, so that there is a problem that the resistance to lateral slipping in the tire can not be enhanced to an expected extent.